The present invention is directed to a variable gain voltage amplifier, and more particularly, to a current conveyor type voltage amplifier circuit that employs a gain dependent biasing scheme to reduce offset and noise to maintain the signal integrity when amplifying a varying input signal.
In the past, different types of circuits have been employed to improve the performance of variable gain amplifiers that employ automatic gain control (AGC) circuits. AGC amplifiers are widely used in communication channels to automatically adjust gain in response to a varying peak to peak swing of an input signal. A typical AGC circuit detects an outputted signal""s peak-to-peak swing, and then adjusts the gain by varying the feedback impedance network which is connected across an opamp to maintain a relatively constant level for the swing of the outputted signal. For example, when an inputted signal swings (rises) to a relatively high level, the AGC circuit will adjust the feedback impedance of the amplifier so that its gain is reduced. Similarly, when the inputted signal""s swing is at a relatively low level, the AGC circuit adjusts the feedback impedance to provide more amplifier gain.
However, an unfortunate side effect of increasing amplifier gain in current conveyor type voltage amplifiers for a faint/weak input signal with a relatively small swing is an increase in noise and the offset voltage due to an increase in the feedback impedance that multiplies noise and offset contributions from the current conveyor""s biasing (current sources) circuits. Thus, for relatively faint/weak signals, the operation of an AGC amplifier can degrade the signal to noise ratio (SNR). Accordingly, it would be advantageous to provide a variable gain amplification circuit that retains the benefits of an AGC circuit without degrading the SNR for weak/faint input signals with a relatively small swing.
The present invention is directed to an apparatus for amplifying a signal. A variable feedback resistance is coupled between the input and output terminals of an opamp. The input terminal of the opamp is connected to the output of the current conveyor circuit (the high impedance node). The input signal is converted into input current by a fixed resistor which connects to the input of the current conveyor circuit (low impedance node). The signal current then is conveyed (directed) to the input of the opamp and, through the feedback resistor to produce output voltage. Due to the high output impedance of the current conveyor circuit, the feedback factor of the amplifier remains close to unity and thus, operating at a loop gain which is close to the open-loop performance. This also results in reduction of power consumption by the opamp since the feedback factor remains close to unity across the AGC range. The current conveyor circuit consists of sink and source currents that are provided partly to bias the input signal swing current. In particular, the source current is scaled such that for a given input swing current, sufficient biasing current is available to maintain signal integrity. The sink current is scaled according to the source current to maintain the biasing conditions. The (sink and source) current sources produce noise and offset contributions that are proportional to the Gm (transconductance) of the transistors. To minimize the offset and noise for a given drain-to-source voltage the transistors are biased such that minimum Gm is achieved while the transistors remain in saturation region. The sink and source current sources are then scaled by the gain control setting of the amplifier to provide sufficient current for the required input current swing. In other words, when the input swing current is at minimum (maximum feedback resistance), the current sources provide minimum bias currents such that maximum SNR is achieved. On the other hand, when the input swing current is at maximum (minimum feedback resistance), the current sources provide large bias currents such that the overall signal-to-noise ratio is relatively constant. Thus, the SNR remains constant across the range of the ADC.
For one embodiment, the invention is directed to at least one switch for disabling at least a portion of the source current source if the high swing of the signal is small. The operation of the switch causes noise produced by at least a portion of the source current source to be reduced. Also, at least another switch is directed to disabling at least a portion of the sink current source if the low swing of the signal is small. The operation of this switch causes noise produced by at least a portion of the sink current source to be reduced.
For yet another embodiment, the invention is directed to at least one of a plurality of PMOS transistors that is employed by the source current source to provide the selected amount of current. Also, at least one of a plurality of NMOS transistors is employed by the sink current source to provide the selected amount of current.
For yet still another embodiment of the invention, the controller includes an automatic gain control component. Also, the controller can be used to determine the size of a peak-to-peak swing in the signal.